1. Field of the Invention:
This invention relates in general to earth boring drill pipe, and in particular to hardfacing the connection members on the drill pipe.
2. Description of the Prior Art:
The most common drill pipe used in earth boring operations have connection members or tool joints on each end that are larger in diameter than the drill pipe. Annular bands of hardfacing are commonly deposited on each tool joint. One type of hardfacing has macroscopic sintered tungsten carbide granules within an alloy steel matrix. Sintered tungsten carbide granules, as explained in U.S. Pat. No. 3,800,891, comprise microscopic grains of tungsten carbide held together by a binder of an iron group metal, usually cobalt. Sintered tungsten carbide hardfacing is normally applied on tool joints by rotating the tool joint, providing an arc with a consumable steel wire, discharging an inert gas around the wire, and gravity feeding sintered tungsten carbide granules into the weld puddle behind the wire.
One disadvantage of the resulting sintered tungsten carbide hardfacing is that many of the granules remain only partially embedded in the matrix, giving a rough, abrasive exterior. In deep wells, intermediate strings of casing are set as the well is drilled. While drilling deeper through a string of intermediate casing, the rough surface of the hardfacing can abrade and damage the casing. Consequently, it is advantageous to have a hardfacing surface free of protruding tungsten carbide granules. In pending application Ser. No. 790,795 filed Apr. 25, 1977, entitled "SURFACE SMOOTHED TOOL JOINT HARDFACING", a method and apparatus is shown for hardfacing tool joints using tungsten carbide granules that results in a smooth exterior. The granules are dropped directly into the arc, rather than behind the arc. This concentrates the granules toward the bottom of the hardfacing deposit and results in a harder matrix near the bottom, as well.
One problem of this method occurs at the tie-in of the weld. The arc creates a generally circular puddle. It is oscillated less than the width of the groove or band to avoid overflow of hardfacing onto the other grooves. This results in a curved initiation point as seen in a side elevational view. To avoid a bare spot, the initiation point is overlapped slightly. This creates a slight protuberance at the tie-in. During the overlapping, the granules continue to fall unitl the arc is stopped, and shortly thereafter. As a result, granules fall into the puddle as it cools, and become embedded in the surface of the deposit. It is desired to have a smooth surface hardfacing of uniform thickness. Grinding this protuberance is difficult because of the granules embedded in the surface. For example, grinding of the protuberances of a typical tool joint may take 10 to 15 minutes, while the hardfacing application may take only 6 to 8 minutes.